Caption: Rare bubble chamber photograph of the production & decay of a neutral xi particle. A negative kaon enters from below & interacts with a proton in the bubble chamber to give rise to several particles. One is the xi-zero, which does not leave a track because it is neutral; the 2 particles into which it decays, a neutral lambda & a neutral pion, are also unseen, but reveal themselves by their decay products. The pion decays immediately into the tightly curving tracks of an electron/positron pair; the lambda produces the V-shaped pair of tracks (negative pion & proton) at upper centre.